This invention relates to electrostatic printing machines and more particularly to an improved technique for determining the voltage level and dark decay rate on the photoreceptor in a printing machine.
Generally, in the process of electrostatographic printing, a photoconductive insulating member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconducting insulating layer is thereafter exposed to a light image of an original document to be reproduced. This records an electrostatic latent image on the photoconductive member corresponding to the information areas contained within the original document. Alternatively, in a printing application, the electrostatic latent image may be created electronically by exposure of the charged photoconductive layer by an electronically controlled laser beam. After recording the electrostatic latent image on the photoconductive member, the latent image is developed by bringing a developer material charged of opposite polarity into contact therewith. In such processes, the developer material may comprise a mixture of carrier particles and toner particles or toner particles alone. Toner particles are attracted to the electrostatic latent image to form a toner powder image which is subsequently transferred to copy sheet and thereafter permanently affixed to copy sheet by fusing.
In reproduction machines using a drum type or an endless belt type photosensitive surface, the surface can contain more than one image at one time as it moves through various processing stations. The portions of the photosensitive surface containing the projected images, referred to as image areas, are usually separated by a portion of the photosensitive surface called the interdocument space. After charging of the photosensitive surface to a suitable charge level by a scorotron, the interdocument space area of the photosensitive surface is generally discharged by a suitable lamp to avoid attracting toner particles at the development stations.
Various portions of the photosensitive surface, therefore, will be charged to different voltage levels. For example, there will be the high voltage level of the initial charge on the photosensitive surface, a selectively discharged image area of the photosensitive surface, and a fully discharged portion of the photosensitive surface between the image areas.
In multi-color electrophotographic printing, in addition to forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are additionally recorded thereon. Each single color electrostatic latent image is developed with toner particles of a color complementary thereto. The process is repeated with a plurality of cycles for differently colored images and their respective complementarily colored toner particles. Each single colored toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. In transferring multiple toner images, each tone image must be in superimposed registration with one another in order to produce a color copy which is not blurred.
Copy sheet quality is dependent on careful control of photoreceptor surface potential. A useful tool for measuring voltage levels on the photosensitive surface is an electrostatic voltmeter or electrometer. The electrometer is generally rigidly secured to the reproduction machines adjacent the moving photosensitive surface and measures the voltage level of the photosensitive surface as it traverses the electrometer probe.
Exact voltage levels on the photosensitive surface, particularly at the developing zone, are necessary for good print quality. Two components of print quality, namely print contrast and background cleanness, are directly affected by the surface potential of the photosensitive surface at the developing zone. The surface voltage is a measure of the density of the charge on the photoreceptor, which is related to the quality of the print output. In order to achieve high quality printing, the surface potential on the photoreceptor at the developing zone should be within a precise range.
Locating a voltmeter directly in the developing zone is one way of measuring the surface potential at the developing zone. However, the accuracy of voltmeter measurements can be affected by the developing materials (such as toner particles) such that the accuracy of the measurement of the surface potential is decreased. In addition, in color printing there can be a plurality of developing areas within the developing zone corresponding to each color to be applied to a corresponding latent image. Because it is desirable to know the surface potential on the photoreceptor at each of the color developing areas in the developing zone, it would be necessary to locate a voltmeter at each color area within the developing zone. Cost and space limitations make such an arrangement undesirable.
An alternative method is to place a single electrometer outside the development zone and use it to monitor the surface potential of the photoreceptor. Such an approach requires a means for relating the voltage which is read by the remotely located electrometer to the voltage on the photoreceptor when it reaches the development zone. In general, there will be a difference, or error, between those two voltages; and that error will increase as the distance between electrometer and development zone increases. Furthermore, the error magnitude is expected to be different for each development zone in the system.
This invention describes a method for estimating that error without using another voltmeter, and, from time to time, revising the error estimate `in situ` in the machine. This invention also may be applied for other purposes, such as diagnostic purposes, when the change in photoreceptor surface voltage with time is of interest.
U.S. Pat. No. 4,355,885 to Nagashima discloses an image forming apparatus having a surface potential controlled device wherein a magnitude of a measured value of the surface potential measuring means and an aimed potential value are differentiated. The surface potential control device may repeat the measuring, differentiating, adding and subtracting operations, and can control the surface potential within a predetermined range for a definite number of times.
U.S. Pat. No. 4,433,298 to Palm discloses a calibrated apparent surface voltage (ASV) apparatus which provides measurements of the ASV on a photoconductive imaging medium by using an ASV probe. A method of measuring an ASV on the photoconductor comprises the steps of a) providing a probe which is responsive to the ASV on an imaging member, b) exposing the probe to both a reference potential and to the ASV of the photoconductor surface so as to obtain a differential probe voltage output during a measurement interval, and c) recalibrating the probe sensitivity during a calibration interval.
U.S. Pat. No., 4,433,297 to Buchheit, assigned to Xerox Corporation, discloses an electrometer probe located adjacent a photosensitive surface. The electrometer head provides an input amplifier which functions as a comparitor to compare a voltage level on the photosensitive surface with a variable high voltage DC power supply. A measuring technique is used to provide a reliable voltage level signal by using a timed average amplitude comparison technique.
While the above-mentioned devices provide for measuring surface voltages, there continues to be a need for an apparatus and method for accurately determining surface potentials, particularly for a plurality of locations on the photoreceptor surface (such as for use in color copying).